Why am I losing so much weight?

I am a 20 year old male. I am just under 6ft tall. A few weeks ago, I was 144 lbs but I suddenly lost 8lbs and dropped to 136 over just the weekend. I was able to gain 4 lbs over the next 2 weeks to reach 140, but over the past week I have dropped back down to 136lbs. I should note that I did suffer a sprained knee and some cuts and bruises last week. I know fluctuation in weight is normal but i feel this is more than just everyday change. I am not on any special diet and, although I am fairly active, I do not have an exercise regimen.